Nutrient Management Research Database

The high-value, large-scale crop production systems in the San Joaquin Valley (SJV) of California typically entail intensive tillage and large fertilizer and water inputs but few C additions to the soil. Such practices often contribute to a decline in soil quality. Our objective for this participatory study was to examine the effects of supplemental C management practices (SCMPs) on various soil quality indicators. To increase farmer participation, we conducted the study on farms using a variety of SCMPs, including cover crops, compost and manure amendments, and several different crop rotations common to the region. The SCMPs significantly changed a number of soil properties, including soil organic matter (SOM); total Kjeldahl N; microbial biomass C and N; exchangeable K; Olsen P; and extractable Fe, Mn, and Zn. A comparison including previously established, adjacent organic, conventional, and transitional fields in addition to the treatment fields at one farm revealed significant differences in 16 of 18 soil quality indicators. A soil quality index computed for this farm scored the established organic system significantly higher than the conventional system. Our results suggest that significant changes in several soil quality indicators occur with a variety of SCMPs. This is especially noteworthy considering the intensive tillage, irrigation, and hot, semiarid environment of the SJV, California, where increases in SOM and related soil properties are generally not expected in a 3-yr study.

Side-by side comparisons of conventional and organic-based production systems were established on 11 farms in autumn of 1995.

• Adjacent fields at each farm were randomly designated to receive either conventional or alternative treatments.
• Field size ranged from 30 to 60 ha.
• Treatment integrity was maintained for 3 years on seven of eleven farms and only those results were presented.
• For the alternative systems, supplemental C management practices (SCMPs) including cover crops, compost and manure amendments were implemented.
• All other practices for each field pair were identical.
• A soil quality index was created to gauge the effects of SCMPs on soil health.

• Significant changes in several soil quality indicators occurred with a variety of SCMPs tested.
• Soil organic Matter (SOM) was higher in the alternative SCMP systems at 4 of 6 fields compared after the final year of treatments.
• Tillage resulted in a downward trend in mean soil C/N ratios.
• Exchangeable K was significantly higher on alternative fields compared with conventional fields in five of six farms in the final year.
• Extractable Fe, Mn, and Zn were significantly higher in soils from alternative fields as well.
• Organic systems received the highest Soil Quality Index scores.